When a vehicle, such as a car, van or truck, tows a trailer there is a tendency for the trailer to oscillate or “fish-tail” behind the vehicle. This can be particularly problematic at high road speeds and during critical manoeuvres such as lane changes and sharp cornering. If left unchecked such oscillations can be dangerous and are a common source of road accidents.
Vehicle-trailer combinations typically act like damped systems in which the magnitude of a damping co-efficient associated with the system dictates the rate at which oscillations of the trailer decay after it has been displaced from a neutral position behind the towing vehicle, e.g. by a gust of wind, etc. With such a system, the greater the vehicle's road speed, the lower the damping co-efficient of the vehicle-trailer system becomes. Thus, the system becomes increasingly unstable as velocity increases. This means that the faster the vehicle travels, the greater the tendency for dangerous and uncontrollable trailer oscillations to occur. This problem is exacerbated by the fact that, in recent years, increasingly stringent vehicle emission regulations have resulted in a decrease in average vehicle weight and studies have shown that this decrease in vehicle weight has had an adverse effect of vehicle stability, particularly when towing a trailer.
Many vehicles are provided with Electronic Stability Control (ESC) (also known as Electronic Stability Programs (ESP) and Dynamic Stability Control (DSC)) which help improve vehicle stability. Such systems typically detect excessive vehicle yaw using gyroscopic sensors or similar arrangements and then counteract this by selectively braking individual wheels of the vehicle. When such a vehicle tows a trailer, ESC can help to improve the stability of the vehicle-trailer system in a similar fashion. However, due to the greatly increased instability and weight of the vehicle-trailer system (relative to the vehicle on its own), a greater magnitude and frequency of braking must be applied to maintain stability, which has a tendency to prematurely wear and overheat the vehicle's brakes. Furthermore, at high road speeds, by the time the oscillations of the trailer reach such a magnitude that they induce sufficient vehicle yaw so as to cause the ESC to apply the vehicle's brakes, it may be too late to take sufficient corrective action with the result that a critical instability in the vehicle-trailer system develops, causing a road accident.
It is an aim of the present invention to substantially overcome or mitigate at least some of the above-mentioned problems.